1) Field of the Invention
The present invention relates to a power umbilical, a DEH power cable or general umbilical, including a plurality of high power cables for transfer of large amounts of electric energy, filler material in the form of stiff elongate plastic elements located at least partially around and between the high power cables and which together are collectively gathered in a spiraled bundle by means of a laying and closing operation, and a protective sheath that encapsulates the high power cables and the filler material.
Thus the present invention can be used on at least three main types of umbilicals/power cables; primarily on that we call a power umbilical of the nature described in WO 2005/124213; on a DEH cable (Direct Electric Heating) of the type described in PCT/NO2010/000395 and on a more traditional umbilical of the nature described in WO 2005/124095.
2) Description of the Related Art
As the oil and gas industry looks for ways to keep pace with the growing worldwide demand for oil, subsea developments are moving into ever increasing water depths and operators are looking towards subsea boosting technology as a means of getting the most out of their reservoirs. The power distribution umbilicals, which are needed to supply electrical power to the subsea boosting pumps, will be challenged by more extreme tension loads due to the deeper waters.
The traditional way to limit the strain in umbilicals is to increase the axial stiffness by adding steel armouring. But for very deep water applications, it soon became obvious that steel as armouring material would not work efficiently to reduce strain of an umbilical. If a structural member suspended from a top point is hanging vertically and exposed to its gravity load only, the strain close to the hang-off point is proportional to the free hanging length and density, but the opposite applies in relation to its stiffness. It is therefore obvious that materials used for stiffness enhancement lose effectiveness as the water depth grows. At a certain water depth the added weight from steel armouring increases the strain rather than reducing it. The problem of using steel as armouring material will typically start in water depths of 2000-2500 meters.
This has made the industry to look for light but still stiff material for umbilical stiffness enhancement. As a result of many years of development and testing, the present applicant has developed and patented a carbon fibre rod reinforced umbilical system. The carbon fibre rods, which are bundled into the umbilical alongside the other elements in the umbilical, have about the same stiffness but only one fifth of the density compared to steel. This change has eliminated all practical water depth limitations in terms of strain and stress induced by the gravity loads.
The control of strain is particularly important for the high and medium voltage electrical power umbilicals. A typical safe long term strain limitation of high power cables is in the range of 0.15% and this can easily be achieved for dynamic high power umbilicals using the carbon fibre rod system. The efficiency of the carbon fibre rod system has been proven in recent projects in water depths of 3000 meters.
The umbilical design of the applicant is based upon the bundling of the internal elements in a gradual spiral along the length of the umbilical. In some cases, and if necessary as an electrical requirement, a second layer is bundled over the inner bundle with the opposite spiral rotation. The umbilical will in such cases have a spiral core and a spiral layer that rotate counter-wise to each other along the length. The angles of the spiral relative to the centre line of the umbilical range between one to three degrees only. This gradual spiral provides several advantages, such as low rotational forces, and a high capacity to withstand axial compression forces of the umbilical.
The internals of the umbilical are held in place and separated by stiff plastic spacers, also called filler material, that run along the length of the umbilical. The spacers are shaped such that when bundled together they form internal longitudinal voids or cavities through which the tubes, or pipes, and cables pass. Each void is exactly dimensioned to suit the internal elements such as a tube or cable that it will contain, with an all round gap of approximately 1 mm. This provides the free movement within the umbilical during handling, and allows then umbilical to be spooled and reeled to the respective bending limits of flexibility of the internal elements.
The use of plastic spacers, the filler material, also provides benefits in the distribution of forces throughout the cross section of the umbilical when it is subjected to high squeeze pressures. Such a benefit is especially advantageous for deep water installation, as it helps prevent the caterpillar grip pressures causing damage to the umbilical internals.
Another important feature of the umbilical of the applicant having bundled elements in gradual spiral along the length of the umbilical is the improved fatigue life due to the low friction between the components.
Usually the umbilical of the applicant is sufficiently stiff and requires no additional stiffening along its length. But in some circumstances, especially in very deep waters and for high power cables, the umbilical needs to be reinforced to restrict excessive strain and stresses upon the bundle elements. Use of steel armouring just below the external sheath that encapsulates the bundle is unsuitable for two reasons; a) the weight of the armouring and hence the tension increases in proportion to the water depth, and b) the compressed spiral weave of the armouring results in loss of stiffness. Therefore in such cases, the umbilical of the applicant is reinforced with carbon fibre rods. The rods are bundled into the umbilical in the same manner as the internal elements such as tubes and cables. The relative density of carbon fibre is 1.6 times that of water compared to steel which is 7.85 times that of water, and therefore the carbon fibre rods contribute their full stiffness without the negative effects of added weight with the use of steel armouring. By using such technology the umbilical can be “tuned” to any depth of water and retain the required stiffness along its length.
One of the main challenges of designing a safe and reliable high power dynamic umbilical for deep water application is to control the forces and strain in the copper cores of the cables. The material properties of copper make it difficult to predict the forces over time as the combination of material creep and temperature effects, i.e. expansion and contraction, in the copper cores makes the evaluation of the fatigue life very uncertain. To minimise the fatigue of the cables in the dynamic bending zone, the applicant has developed a concept with the electrical power cables “free floating” in the dynamic bending zone, thus avoiding any uncontrolled loads upon the cables in the zone. The other load bearing elements of the umbilical bundle are; the steel tubes (if any) and carbon fibre rods are anchored at the top hang-off, but the electrical cables run through the hang-off without any constraint and are therefore free to expand and contract.
Instead of a topside cable termination, the cables are hung-off inside the umbilical with a “soft clamp system” according to the present invention. In one embodiment the “soft clamp system” is basically a vulcanized rubber sheath that is applied to the outside of the cable over a length of about ten meters, increasing the outer diameter and providing friction against the inside of the cable conduit in the umbilical, without creating a “hard point” between the surfaces.
Such “soft clamp” is located some 15 to 20 meters below the bend stiffener, and becomes the hang-off point of the power cables. From this point and up through the dynamic bending zone, the cables are free to expand, and are exposed to bending only, and therefore the forces upon the electrical cables become highly predictable. In one embodiment, descending below the hang-off point, the power cables are secured by frictional points in a special pattern over a length of about two meters at intervals of about fifty meters to counteract any long term creep/deformation of the copper cores. The “soft clamp system” protects the electrical cables from strain hardening and fatigue failure in the dynamic section.
Normally, as indicated above, the strength of a cable is an important issue when it comes to deep waters, substantial spans or under different circumstances when the cable is subjected to high loads. Phenomenas that are not so easy detectable also occurs. Due to the temperature generated in cables during operation and long term creeping of copper, a dynamic power umbilical is designed on that basis that the power conducting cables shall not contribute to any axial stiffness, only by its weight. This will be further detailed later in this description.